Scale and snapping ... something doesn't work
The maneuver in your illustration is a constrained scale along global X, with the 3D cursor as pivot.  I'm not sure how the constraint was applied; I would expect to see it in the keystrokes or in the use of X axis of the scale manipulator.

Anyway, the settings show: "snap the 'Closest' element (vertex) of your transforming  selection to a 'vertex' on the target, (chosen by hovering over it), while transforming the rest of the selection with it"

So the moves are: 


Ensure the pivot is at the center of your scale. That's the point which won't move
Select the vertices you want scaled away from the pivot
Hit SX, or use the manipulator to constrain the scale to X
Hover over the target vertex
Left-click with your cursor inside the little snap target circle to confirm.


'Closest' seems to select the vertex of the transforming selection that was closest to the hover target along the constraint axis before the transform began as the point to lock on the the target. At least, it does when the entire transforming selection was to one side of the target down that axis before the move was made. However I have to say I find its behavior unpredictable when the target is between or inside the source selection.

So I would much rather take the little extra time to use 'Active' as the source element. Then, in vertex mode, you know exactly which vertex will be aligned with your target by ensuring it is the last selected. That may involve shift right-clicking it twice.